[codec][vaapi] Add const to copy assignment

llvm/llvm-project#89652 made
std::pair trivially copyable if its members are, and
it caused the following issue: "error: the parameter
for this explicitly-defaulted copy assignment operator
is const, but a member or base requires it to be
non-const". This patch adds const to copy assignment
operator to fix the issue.

Bug: 354627057
Change-Id: I10ee04c40da8820e33b80f51492e136a27e97972
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1086537
Fuchsia-Auto-Submit: Gulfem Savrun Yeniceri <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>
Reviewed-by: John Bauman <[email protected]>

src/media/codec/codecs/vaapi/codec_adapter_vaapi_encoder.h

@@ -47,7 +47,7 @@ class VaApiEncoderOutput {
   ~VaApiEncoderOutput();
 
   VaApiEncoderOutput& operator=(VaApiEncoderOutput&& other) noexcept;
-  VaApiEncoderOutput& operator=(VaApiEncoderOutput& other) noexcept = delete;
+  VaApiEncoderOutput& operator=(const VaApiEncoderOutput& other) noexcept = delete;
 
  private:
   uint8_t* base_address_ = nullptr;